The present invention relates to a method and to apparatus for unplugging a well or a pipe which is obstructed by a solid plug of gas hydrates.
In the oil trade or industry, a problem sometimes arises associated with the appearance of solid plugs inside a borehole or a pipe for transporting oil products, in particular pipeline or a feed pipe.
In the present description, the terms xe2x80x9cupstreamxe2x80x9d and xe2x80x9cdownstreamxe2x80x9d when relating to a well or pipe correspond to the direction in which the oil product flows in the well or in the pipe.
Thus, when referring to an oil well, for example, its upstream end is level with the hydrocarbon reservoir, while its downstream end is at the surface.
Hydrate plugs are observed to form in wells or pipes in which the internal bore is at a pressure which is very high and at a temperature which is low.
This occurs in particular in off-shore wells.
Hydrates are substances made up essentially of water, together with various chemical substances, and having a melting temperature of about 0xc2x0 C. at atmospheric pressure.
This melting temperature increases relatively fast with pressure. Thus, by way of indication, this melting temperature is about 10xc2x0 C. at a pressure of 20 bars, 15xc2x0 C. at a pressure of 50 bars, 20xc2x0 C. at a pressure of 100 bars, and 22xc2x0 C. at a pressure of 200 bars.
Still by way of indication, in the North Sea, where there are to be found numerous oil-bearing deposits in production, the temperature is substantially constant and about 8xc2x0 C. to 9xc2x0 C.
The pressure that exists in the wells or pipes often lies in the range 50 bars to 300 bars, and consequently corresponds to hydrates existing in the solid state.
To prevent such solidification, it is general practice to insert antifreeze liquids, in particular methanol, into wells or pipes for the purpose of lowering the melting point of the mixture very significantly.
Although that method generally gives satisfaction, it can nevertheless happen accidentally that solid hydrate plugs appear, thereby obstructing the well or pipe and preventing it from being used.
So far as the Applicant is aware, there does not exist at present any method or apparatus that is really adapted to eliminating such plugs.
Two methods are used.
The first consists in expelling hydrate plugs by inserting a fluid under very high pressure into the well or the pipe.
That solution is effective in certain cases only.
Another method consists in drilling through the plug(s), thereby making it necessary to use equipment that is extremely sophisticated and expensive, while nevertheless compromising the integrity of the well or the pipe; there is a risk of the drilling tool damaging the walls of the well or the pipe during the operation.
That is why the present invention proposes filling this void, by proposing a method and apparatus that enable the well or the pipe to be unplugged in a manner that is simple and of low cost, completely safe, and without running the risk of damaging the walls.
The method of the invention for unplugging a well or a pipe obstructed by a solid hydrate plug and which enables this object to be achieved is remarkable in that a moving heating element is pressed against one end of said plug, and is displaced axially in the tube or pipe towards the other end so as to cause the plug to melt progressively from one end to the other.
In one possible implementation of this method, during the operation, an xe2x80x9cantifreezexe2x80x9d liquid is supplied for lowering the melting temperature of the hydrate plug inside the well or the pipe in the vicinity of the end of the plug against which the heating element is applied.
Advantageously, the heating element is electrically heated.
According to another characteristic of the method, the heater element is displaced under the effect of gravity.
Naturally, this assumes that the well or the pipe extends in a vertical direction or at least slopes with a significant vertical component.
In a variant, the element is displaced by thrust, with the thrust action being assisted or otherwise by the effect of gravity.
In a preferred implementation of the invention, a moving heating element is applied against one end of said plug while an antifreeze liquid is caused to circulate inside the well or pipe in a closed circuit in the vicinity of said end while simultaneously displacing the heating element axially towards the other end of the plug in such a manner that the heat delivered by the heating element and the presence of the antifreeze liquid together ensure that the plug melts progressively and irreversibly.
In which case, said heating element advantageously includes electrical resistances for providing heating by the Joule effect, while the antifreeze liquid is circulated by means of an electric pump, and the electrical power for said electrical resistances and the pump is delivered from surface equipment by means of conductors located in a suspension cable or rod.
The apparatus which also forms part of the present invention and which serves to unplug a well or a pipe obstructed by a solid hydrate plug is remarkable in that it comprises a moving heating element adapted to be inserted in and displaced axially along the well or the pipe in such a manner as to be applied against one end of said plug and to cause it to melt progressively from one end to the other while being displaced towards the other end.
Furthermore, according to various additional but non-limiting characteristics of the invention:
said heating element is a metal body that conducts heat and that houses electrical resistances that are electrically powered from surface equipment;
the apparatus includes means for injecting an antifreeze liquid into the well or the pipe in the vicinity of the hydrate plug;
the apparatus includes means for continuously recycling the antifreeze liquid;
said heating element is an elongate body whose free end for being applied against the hydrate plug is of generally tapering shape; and
the apparatus is suspended from a cable connected to the surface equipment, and it is lowered by gravity inside the tube or the pipe, said cable containing various electrical conductors enabling the apparatus to operate, in particular to power the heating resistances and, where appropriate, the pump which circulates the antifreeze.
In a preferred embodiment, the apparatus comprises:
a) a moving heating element constituted by an elongate metal body that conducts heat, the free end of the body for application against the hydrate plug being of a generally tapering shape;
b) electrical resistances housed in the body and suitable for heating it by the Joule effect;
c) an electric pump secured to the heating element and adapted to cause an antifreeze liquid to circulate in a closed circuit around the tapering end; and
d) a cable or rod to which the heating element and the pump are fixed and via which they are connected to surface equipment, said cable or rod containing electrical conductors for electrically powering the resistances and the pump.